Trouble with casting array of objects back to an array of integers

Question

When I go to run this I get the error:

Exception in thread "main" java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [Ljava.lang.Integer; at Main.main(Main.java:30)

public class Main 
{ 
    private interface Function<R, D> 
    { 
        public R apply(D parameter);
    }

    public static void main(String[] args) 
    {           
        // Example 1
        Function<Integer, Integer> function = new CalculateSuccessor();
        Integer[] integerArray = {1, 3, 4, 2, 5};
        PrintArray(map(function, integerArray)); // map returns {2, 4, 5, 3, 6}  <--- line 30
    }

    @SuppressWarnings({"unchecked"})
    public static <R, D> R[] map(Function<R, D> function, D[] array)
    {
        R[] results = (R[]) new Object[array.length];

        // Iterate through the source array, apply the given function, and record results
        for (int i = 0; i < array.length; i++)
        {
            results[i] = (R)function.apply(array[i]);
        }

        return results;  
    }

    public static <T> void PrintArray(T[] array)
    {
        for (T element : array)
        {
            System.out.println(element.toString());
        }   
    } 
}

Presumably line 30 is the one that says R[] results = (R[]) new Object[array.length]; ? — Dawood ibn Kareem
– Dawood ibn Kareem, Commented Oct 3, 2013 at 3:23
Line 30 is when I call the PrintArray method. My apologies for not stating that. — Jake Klapper
– Jake Klapper, Commented Oct 3, 2013 at 3:29
My guess is that R is being resolved to Object, not to Integer as you expect. Honestly, arrays and generics don't usually play nicely together. If I had to write this, I would have made map return a List<R> instead of an R[] - it just seems cleaner somehow. — Dawood ibn Kareem
– Dawood ibn Kareem, Commented Oct 3, 2013 at 3:44
I ran the code with my own Function(which just returns the parameter) it ran fine. intellij 12 - jdk1.7.0_21 — BevynQ
– BevynQ, Commented Oct 3, 2013 at 3:54
It may be that the issue is in the definition of CalculateSuccessor - maybe this constrains R somehow. — Dawood ibn Kareem
– Dawood ibn Kareem, Commented Oct 3, 2013 at 4:01

Community · Accepted Answer · 2017-05-23 11:50:04Z

0

Problem is the result array is created as new Object[array.length]. The cast (R[]) doesn't convert this array to a different type, it just tells the compiler to treat it like R[]. See Quick Java question: Casting an array of Objects into an array of my intended class. Creating an array of generic type is a bit nasty - you can google for various solutions, e.g. How to create a generic array in Java? - but generally the simplest approach is to use a list instead of an array: you can create it with new ArrayList<R>

edited May 23, 2017 at 11:50

CommunityBot

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answered Oct 3, 2013 at 3:59

Mike Stockdale

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